The invention relates to optical communication systems, and more specifically, to devices and methods for selecting spatial modes in optical signals.
Optical signals transmitted through an optical communication fiber are subject to chromatic dispersion, which limits the rate at which data can be transmitted between two points. One or more optical fibers having high negative dispersion, known as dispersion compensation fibers (DCFs) can be used to compensate for the dispersion effect of the communication fiber. Methods and apparatus relating to DCFs are described in pending U.S patent application Ser. Nos. 09/249,830, 09/248,969 and 09/249,920, each filed Feb. 12, 1999, whose contents are incorporated herein by reference and which are assigned to the assignee of this application. In one concept disclosed therein, in order to reduce the length of optical fiber necessary for such compensation and to compensate for the dispersion slope, it is desirable to transform the optical signal from a lower order spatial mode (e.g., LP01) to a higher order spatial mode (e.g., LP02) using a transverse spatial mode transformer before transmitting the optical signal through the DCF. The DCF has been designed to function optimally with a higher order spatial mode. The signal is then transformed back to a lower order spatial mode after exiting the DCF using another transverse spatial mode transformer. The performance of the dispersion compensation device is limited in part by energy lost in unwanted spatial modes resulting from impurities in the compensation fiber, fabrication tolerances in the transverse spatial mode transformers, and alignment of the device with respect to the communication fiber.
Uses for higher order spatial modes include special transmission fibers, such as the one described in pending U.S. Provisional Patent Application 60/121,321 filed Feb. 23, 1999.
The use of optical elements to produce an optical coordinate transformation has been discussed in the prior art. Two exemplary articles include Bryngdahl, O. xe2x80x9cGeometrical Transformations in Optics.xe2x80x9d Journal of the Optical Society of America (August 1974):1092-1099., and Davidson, N.; Fresem, A.A.; and Hasman, E. xe2x80x9cOptical Coordinate Transformations.xe2x80x9d Applied Optics (March 1992):1067.
U.S. Pat. No. 5,760,941 discloses an optical mask utilized as an amplitude mask, which allows certain spectral frequencies to pass.
U.S. Pat. No. 4,895,421 discloses the use of a mask to attenuate optical energy radiating from one of two lobes of the LP11 spatial mode. This acts to assist in the detection of an on or off condition caused by the activation or deactivation of a pump signal.
However none of the above references teach a method for removing unwanted spatial modes remaining, or describe a profile for converting in a bi-directional manner spatial modes existing in an optical waveguide to a different spatial mode existing in another waveguide.
The invention relates to an apparatus for transforming a spatial mode of an optical signal. The apparatus includes an optical phase element for imparting a predetermined spatially selective phase delay to the optical signal, and a mask in optical communication with the optical phase element. The mask, in one embodiment, includes an optically attenuating region disposed in a predetermined spatial pattern. In another embodiment, the mask is integral with the phase element. In another embodiment, the phase element includes a calcium fluoride window.
The apparatus, in one embodiment, includes an optical element in optical communication with the optical phase element. The optical element can be a lens. In another embodiment, the lens is integral with the optical phase element. In another embodiment, the phase element is in optical communication with an optical waveguide. In still another embodiment, the phase element is in optical communication with a second optical waveguide. The apparatus, in another embodiment, includes an optical element disposed between the optical phase element and the optical waveguide. In still another embodiment, an optical element is disposed between the optical phase element and the second optical waveguide.
In one aspect of the invention, the optical signal is transformed from a first spatial mode to a second spatial mode. The first and second spatial modes, in other embodiments, are the LP01 spatial mode or the LP02 spatial mode.
In another aspect of the invention, the mask includes an absorbing material arranged in a predetermined spatial pattern. In yet another aspect of the invention, the mask includes a scattering material arranged in a predetermined spatial pattern. In one embodiment, the optically attenuating region includes a sharp profile. In other embodiments, the predetermined spatial pattern is coincident with a minimal energy point of the first or second spatial mode.
The invention also relates to a method for attenuating an undesired spatial mode in an optical mode transformer. The method includes the steps of receiving the optical signal having the undesired spatial mode with an intensity distribution, selectively spatially attenuating the intensity distribution of the undesired spatial mode, and propagating the selectively spatially attenuated optical signal. The undesired spatial mode is substantially removed from the optical signal. In one embodiment, the undesired spatial mode is the LP01 spatial mode. In another embodiment, the undesired spatial mode is the LP02 spatial mode.
The invention further relates to a method for transforming a spatial mode of an optical signal. The method includes the step of receiving the optical signal having a first spatial mode with an intensity distribution and a phase. The method further includes the steps of selectively spatially attenuating the intensity distribution of the first spatial mode of the optical signal, selectively spatially delaying the phase of the optical signal, and propagating the selectively spatially attenuated and delayed optical signal. The first spatial mode of the optical signal is substantially transformed into a second spatial mode. In other embodiments, the first and second spatial modes are the LP01 spatial mode or the LP02 spatial mode.